Since their discovery both Mycobacterium leprae (agent of human leprosy) and Mycobacterium lepraemurium (agent of murine leprosy) have been regarded as obligate intracellular microbes. In 1972 Nakamura descrived a semi-synthetic system in which M. lepraemurim could initiate a single growth cycle. By optimizing, then further supplementing this system, the continuous growth of this agent has been achieved in this laboratory. The principles which have been identified will now be applied and amplified during attempts to secure the in vitro growth of M. leprae.